The binding of NADH and NADPH to bovine liver glutamate dehydrogenase has been studied using circular dichroism (CD) and fluorescence spectroscopy, under comparable conditions of enzyme concentration. Spectroscopic titrations have been analysed using non-linear least-squares techniques to determine dissociation constants and parameters for the different chromophoric portions of the bound coenzymes, according to different models for the binding equilibria. 2. For NADPH, enhancement of dihydronicotinamide fluorescence and positive CD at 340 nm reflect binding of the coenzyme at the active site (site I) via the dihydronicotinamide moiety with a dissociation constant K1 of 50 microM; negative CD at 260 nm reflects the binding of a second molecule of coenzyme at the non-active site (site II) via the adenine moiety with a KII of 650 microM. 3. For NADH, the same spectroscopic assignment is made; however, site II affinity is much greater (KII 20 microM) and exceeds the affinity at the active site (KI 50 microM). 4. In no case was spectroscopic evidence obtained with either coenzyme to support the concept of non-identical active site binding as three sites IA and three sites IB per hexamer. 5. Coenzyme affinities are approximately twofold higher in triethanolamine buffer (0.05 + 0.1 M KCl, pH 7.6) compared with phosphate buffer (0.1 M at pH 7.0). 6. ADP enhances the CD of bound NADPH at 340 nm by up to twofold; phosphate ion also enhances the CD and binds with a Kd of 60-80 mM. GTP causes strong negative CD at 340 nm but only for the NADPH bound at site II. These results are consistent with independent binding sites for ADP (site II) and GTP (site III); phosphate ion is apparently able to affect certain properties of all three sites.
